In electronic prepress systems, images to be printed by offset printing are scanned from photographic sources, digitized, assembled, and edited electronically at a workstation. The digitized images are then transmitted to a raster image processor (RIP) for half-tone screening and image rasterization. The RIP image, or rasterized image, to be printed is then transmitted from the RIP to an imagesetter for photographic or film recording onto a medium such as paper, film, or a printing plate.
An imagesetter includes a supply of unexposed photosensitive media, a recording support surface, and an image exposing system for forming the image to be recorded according to the RIP image data. The image exposing system may employ a laser beam, a cathode ray tube (CRT), an LED emitter, or the like as a radiation source. The media passes either from single sheets from a supply roll or as a web to the recording support surface at which point the photosensitive media is exposed by the radiation source, forming a latent image on the media. Numerous images may be recorded on the web consecutively. The exposed web is then advanced for transfer to a media processor where chemical processing occurs.
Three inks, yellow, magenta, and cyan, are used to print color images. Often black ink is also used. The inks are printed in small dots, sometimes overlaid, in varying amounts to create the desired colors when viewed. Thus, three or four black and white separation films must be imaged, one for each color.
In the printing process, the films are overlaid and must be aligned accurately to ensure a good quality image. Toward this end, registration openings or holes are punched in each film to serve as an alignment guide. The location of each pixel on each film is determined with respect to the registration holes which are punched along an edge of the media, generally either the leading edge or a side edge. Typically, the openings must be punched with an accuracy of 1 mil with respect to the image on the media.
In some applications, it is desirable to punch registration openings along the side of the media rather than along the leading edge. In internal drum imagesetters, however, there is a small clearance, approximately 0.5 inch, between the imaging surface of the drum on which the sheet of media is supported and the scanning apparatus. Due to this small clearance, prior art internal drum imagesetters have typically not provided side punch capability. The present invention provides an internal drum imagesetter with a low profile side punch to punch registration openings along the side edge of media in an internal drum imagesetter.
More particularly, the low profile side punch assembly includes a punch die mounted to the drum adjacent the side face of the drum. A punch opening is formed through the punch die. A punch actuator is movably, preferably pivotably, mounted with respect to the imagesetter. The actuator comprises a punch receiving arm having an end extending over the punch opening. A punch pin is mounted to the end of the punch receiving arm and aligned to be received in the punch opening. A drive mechanism is operatively coupled to the punch actuator to move the punch actuator with respect to the punch die to move the punch pin within the punch opening, to thereby punch a registration opening in a sheet of media which has been fed into the guide slot.
The side face of the drum includes a slot having a shoulder or other reference surface, such as a T-shaped or trapezoidal slot, formed therein. A correspondingly shaped fastening device is mounted within the slot to adjustably affix the punch die to the side face. During manufacture, the location of the punch opening is set to a determined distance from the side face of the drum, and an upper surface of the punch die is aligned with and parallel to the imaging surface of the drum by the reference surface.
The invention also relates to a method of punching a registration opening in an edge of a sheet of media on an imaging surface of an imagesetter. In the method, a side punch assembly is provided mounted to the side face of the imagesetter A sheet of media is advanced into the imagesetter with an edge of the sheet of media disposed in the side punch assembly. The drive mechanism is actuated to move the punch into the sheet of media.
The method further comprises actuating the drive mechanism to retract the punch pin from the sheet of media. In another step, a position of the punch pin is sensed and the drive mechanism is controlled to prevent retraction of the punch pin before the punch pin is retracted from the sheet of media.